Conventional four-stroke internal combustion engines include one or more cylinders. Each cylinder includes a single piston that performs four strokes, commonly referred to as the intake, compression, combustion/power/expansion, and exhaust strokes. Together, these four strokes form a complete cycle of the engine, carried out during two complete revolutions of the crankshaft.
In a conventional internal combustion engine, each part of the cycle is affected differently by the heat rejected from the working fluid into the piston and cylinder walls: during intake and compression a high rate of heat rejection improves efficiency whereas during combustion/expansion, little or no heat rejection leads to best efficiency. This conflicting requirement cannot be satisfied by a single cylinder since the piston and cylinder wall temperature cannot readily change from cold to hot and back to cold within each cycle. A single cylinder of a conventional internal combustion engine cannot be optimized both as a compressor (requires cold environment for optimal efficiency performance) and a combustor/expander (requires hot environment and optimal expansion of the working fluid for optimal efficiency performance) at the same time and space.
Conventional internal combustion engines have low fuel efficiency—more than one half of the fuel energy is lost as heat through the engine structure and exhaust outlet, without adding any useful mechanical work. A major cause of thermal waste in conventional internal combustion engines is the essential cooling system (e.g., radiator), which alone dissipates heat at a greater rate and quantity than the total heat actually transformed into useful work. Furthermore, conventional internal combustion engines are able to increase efficiencies only marginally by employing low heat rejection methods in the cylinders, pistons and combustion chambers and by waste-heat recovery methodologies that add substantial complexity and cost.
Further inefficiency results from high-temperature in the cylinder during the intake and compression strokes. This high temperature reduces engine volumetric efficiency and makes the piston work harder and, hence, reduces efficiency during these strokes.
Another shortcoming of conventional internal combustion engines is an incomplete chemical combustion process, which reduces efficiency and causes harmful exhaust emissions.
To address these problems, others have previously disclosed dual-piston combustion engine configurations. For example, U.S. Pat. No. 1,372,216 to Casaday discloses a dual piston combustion engine in which cylinders and pistons are arranged in respective pairs. The piston of the firing cylinder moves in advance of the piston of the compression cylinder. U.S. Pat. No. 3,880,126 to Thurston et al. discloses a two-stroke split-cycle internal combustion engine. The piston of the induction cylinder moves somewhat less than one-half stroke in advance of the piston of the power cylinder. The induction cylinder compresses a charge, and transfers the charge to the power cylinder where it is mixed with a residual charge of burned products from the previous cycle, and further compressed before igniting.
U.S. Pat. No. 6,609,371 to Scuderi discloses a four-stroke cycle internal combustion engine. A power piston within a first cylinder (power cylinder) is connected to a crankshaft and performs power and exhaust strokes of the four-stroke cycle. A compression piston within a second cylinder (compression cylinder) is also connected to the crankshaft and performs the intake and compression strokes of a four-stroke cycle during the same rotation of the crankshaft. The power piston of the first cylinder moves in advance of the compression piston of the second cylinder. U.S. Pat. No. 6,880,501 to Suh et al. discloses an internal combustion engine that has a pair of cylinders, each cylinder containing a piston connected to a crankshaft. One cylinder is adapted for intake and compression strokes. The other cylinder is adapted for power and exhaust strokes. U.S. Pat. No. 5,546,897 to Brackett discloses a multi-cylinder reciprocating piston internal combustion engine divided into a working section and a compressor section. The working section supports the combustion function and the compressor is dedicated solely to infusion of intake charge into the working section.
U.S. Pat. No. 8,584,629 to Tour et al., incorporated herein as reference in its entirety, discloses a two-cylinder, double piston combustion engine with an interstage valve for fluidly coupling the two cylinders. In one embodiment the internal volume of the compression cylinder is smaller than the internal volume of the expansion cylinder, thus enabling additional conversion of heat and pressure to mechanical work. In another embodiment the internal volume of the compression cylinder is larger than the internal volume of the expansion cylinder, thereby allowing for a greater amount and/or higher pressure of fuel mixture (i.e., “supercharged” fuel mixture) to be injected into the combustion chamber and, hence, provide more energy and work, during the expansion stroke.